Aircraft fuel naturally contains some dissolved gas, typically air, and therefore fuel typically contains some dissolved oxygen. The amount of oxygen in fuel decreases with pressure. Therefore, at cruising altitude (i.e., at low ambient pressure), oxygen is degassed from the fuel. From a safety standpoint, it is desirable to have fuel or fuel-rich environments contained or embedded in an inert atmosphere. Thus, the release of oxygen from fuel in such an environment is highly undesirable.
Also, evolved gas from the fuel may increase the risk of air pockets forming in the fuel system of the aircraft. Some aircraft fuel-tank arrangements use gravity feed systems including siphons, to transfer fuel within the fuel system. Air pockets present in the fuel tank system may act to disrupt the siphon effect in gravity feed systems, the pressure head possibly being insufficient to push the air down the pipes. Consequently air pockets could be created, which can interfere with fuel flow in the pipes.
A known technology for estimating oxygen concentration in gas and liquid uses electrochemical detection. A prior-art sensor comprises an electrochemical cell with an anode and a cathode in an electrolyte solution. The electrochemical cell is separated by a membrane from a (gas or liquid) sample, the oxygen concentration of which is to be measured. Oxygen diffuses from the sample across the membrane into the electrochemical cell to establish fugacity equilibrium. The fugacity is proportional to the ambient oxygen concentration. A change in oxygen concentration in the electrolyte causes a change in its electrical property and a resulting change in electric current through the system. The current is proportional to the oxygen concentration in the electrolyte. The operating limits and sensitivity of the sensor are characterised by the electrolytes used. However, most commonly used electrolytes are not suitable for extreme operating temperatures. In particular, they are not suitable for the low temperatures encountered in aviation applications.
W A Rubey et al. (Journal of Chromatographic Science, Vol. 33, 1995, 433-437) describe a method for the analysis of atmospheric gases and other small molecules in thermally stressed jet fuel. As with the method employed by V V Malyshev et al. (Trudy, KIIGA, 1970, p 3), the chromatographic method uses packed columns for separation of permanent gases. Although the analytical system of Ruby et al. allows monitoring of fuels in a continuous, flowing, high-pressure stream, the system is overly complex, containing many switching valves and three co-joined columns. It is thus completely impractical for in situ aerospace applications.
A method for the analysis of dissolved gases in insulating oil by gas chromatography is described in ASTM International Designation D3612-02 “Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography” (available from ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, Pa., 19428-2959 USA). However, this method would not be readily applicable to the aviation fuel, as it would not allow for “in-line monitoring” given the complexity of gas chromatography equipment.
Thus, the above prior art does not provide a method or apparatus that enables fast and accurate monitoring of the concentration of oxygen in aircraft fuel or in the fuel tank ullage of an aircraft or aircraft vehicle.
A further proposal of the prior art, disclosed in U.S. Pat. No. 5,919,710 (Gord et al), relates to the measurement of dissolved oxygen in fuel flowing in a fuel line by means of doping the fuel with a luminophore and measuring the phosphorescence of the luminophore when excited by a pulse of laser light (i.e. a coherent light source). The oxygen concentration is related to the decay in light intensity phosphoresced by the luminophore (the luminescence lifetime). The method requires the bulk doping of fuel with a luminophore. Such a method can not therefore be viewed as a viable and practical method of measuring oxygen concentrations in fuel in commercial aerospace applications, as the bulk doping of fuels with such luminophores would be impractical. Such a method also has application only in relation to measuring dissolved gas concentrations in liquids.
The need to provide a coherent light source in accordance with the teachings of Gord et al would appear to envisage the provision of relatively high power laser devices. For example, the pulsed laser light provided in accordance with the teachings of Gord et al would appear to have a peak power density (the peak optical power per cross-sectional area of laser beam) of greater than 10 megawatts per cm2. Such relatively high peak power densities would appear possible (i.e. without significant safety risks) in Gord et al by virtue of the fuel flowing when exposed to the laser light. Whilst such high peak power densities may not cause significant ignition risks in relation to moving fuel, using relatively high powered pulsed laser light in a static fuel environment could give rise to significant safety risks. It would seem that the proposal of Gord et al would not therefore be of practical application either in relation to the measurement of oxygen concentrations in a fuel tank or in relation to the measurement of oxygen concentrations in or next to fuel in commercial passenger aircraft, because of safety and/or aviation certification issues.
The present invention seeks to ameliorate at least some of the above-mentioned problems. Alternatively, or additionally, the present invention seeks to provide an improved apparatus and method for detecting gas levels in aerospace applications. Alternatively, or additionally, the present invention seeks to provide an improved apparatus and method for safely detecting gas levels in aerospace applications, where fuel is present.